Potable water in many cases requires additional purification (afterpurification) in area of consumption, since traditional water-treatment system constructions do not always allow to remove or reduce concentration of individual contaminants up to standardized amounts, which in final result affects quality of potable water.
Generally, an afterpurification system for additional purification of potable water includes a column of activated carbon used to remove chlorine, organic chemicals, and other contaminants. In many cases, activated carbon of granular form is used. Advantages of use of filters with granular activated carbon are that granular activated carbon is a cheap raw material, has rather small hydraulic resistance, and does not consolidate when liquid flows. A disadvantage is low rate of adsorption. Therefore, with use of granular activated carbon in the industry, the recommended time of contact of a sorbent in the column and a liquid is not less than 15 minutes, which amount of time is unacceptable for ordinary living conditions. Besides, a column with granular activated carbon does not remove fine contaminant particles because of relatively large size of the sorbent granules and the free space (voids) between them.
Powdery activated carbon, as it is known, adsorbs impurities faster than granular activated carbon does, however, use of smaller fraction creates high hydraulic resistance and results in undesirable compaction of a filtration column when a liquid flows. To avoid compaction, a binder is usually injected. Filtering elements comprising particles of activated carbon, aggregated in a hard structure by polymeric binder in the form of a thermoplastic material, do not consolidate when a liquid flows. However, in this case, a part of adsorption volume of activated carbon is not used in operation because of contact with an immobilizing binder. In those filters, activated carbon particles occupy only a part of adsorptive column volume, while at the same time another part of the adsorptive column volume is occupied by the binder. The binder is not active to adsorb contaminants, and its introduction results in increasing a column size in comparison with an adsorptive column, which comprises only activated carbon. Besides, manufacturing such filtering elements is expensive and requires exact temperature regimes, and other high-precision controlled process parameters.
Methods of using activated carbon fibers in columns is known, but those methods are rather expensive and cause compaction of the fibers when liquid flows, which results in increased flow resistance, although the fiber compaction is significantly smaller than using powdery activated carbon.
In the prior art, it is known to use filtering elements for liquid purification on the basis of a mixture of granulated material and fibrous material. For example, there is known a composite material consisting of an adsorbing granulated material and a material capable of extending when it contacts with a liquid (U.S. patent application No 20040232068 A1, published on 25 Nov. 2004, IPC B01D 039/100). An adsorbing material can be, for example, a granulated activated carbon (80-325 mesh) and granulated ion-exchange resin, and a polymer extending material can be, for example, ion-exchange fibers based on a polyacrylonitrile. Components of the materials are carefully mixed and located in a suitable case, which case volume limits swelling value of the extending material. Formation of a porous composite material occurs at its contact with a liquid in the this case due to immobilizing the granulated material in an extending material in the course of its swelling, and also due to pressure of internal walls of the case, forming a structure with random distribution of original components that are strongly enough kept together, while at the same time the composite material is capable of taking the case shape. The extending material can be at quantities from 0.1% to 99.9% by weight.
Disadvantages of the described porous composite material consist in that, at stage of obtaining it, namely, at immobilizing granular particles into an extending material, it is impossible to form a substantively ordered porous structure. Besides, in intervals between filtrations, in course of dehydration of an extending material, the granular component can be undesirably moved, and, as a consequence, gradual destruction of the composite porous filtration structure can occur.
Another example of use of a composite material for liquid purification, in particular, for potable water, based on a mixture of granular and fibrous materials, is described in Russian Federation patent No 22132729, published on 10 Jul. 1999, IPC B01D 039/02, and U.S. Pat. No. 6,299,771, published on 9 Nov. 2001, IPC B01D 039/00.
Russian Federation patent No 2132729 discloses the composite adsorptive material comprising a packed mixture of granules and fibers in the form of granular activated carbon in the amount of 5-30% by weight and activated carbon fibers in the amount of 70-95% by weight. The mixture also can be consisted of ion-exchange granules and ion-exchange fibers. In the composite adsorptive material, the fibers closely fill the space between activated carbon granules. Sizes and relative amounts of granules and fibers are selected so that the filtering element filters water of contaminants faster than traditional filters with granules of activated carbon. At significant time periods between filtrations, the packed mixture of granules and fibers indicates improved characteristics and recovery of adsorptive capabilities during intervals between filtrations.
A filter described in U.S. Pat. No. 6,299,771 consists of a compacted mixture of components: granulated activated carbon in the amount of 5-30% by weight and the activated carbon fibers in the amount of 70-80% by weight. The mixture of ion-exchange granules with ion-exchange fibers also can be used in the filter. As granules, particles with the size of 50-3000 μm, preferable 100-2000 μm are used. The length of fibers is 0.2-20 mm, preferably 1-10 mm, and the average diameter is 1-30 μm, more preferably 4-20 μm. Average diameter of granules is to be more than average diameter of the fiber. At the same time, fibers are intertwisted among themselves in the composite structure and around closely packed granules of activated carbon adjoining with each other, and with high grade of filling voids between the granules. As a result, numerous channels for water passage are formed, at the expense of which the filtering element hydraulic resistance is reduced. The described composite material is capable of restoring adsorption properties (prototype) in the intervals between the filtrations.
Aforesaid examples of filtering elements made on the basis of composite have a number of disadvantages. As disclosed in Russian Federation patent No 2132729 and in U.S. patent of the U.S. Pat. No. 6,299,771, the fibers of composite material structure do not provide sufficient fixation of small fractions of the granular component, and undesirable compaction of the composite material can occur in the course of water flowing through the filtering element, resulting in increased filtration time and reduced filtration rate of the filtering element, and also damage of composite material structure because of capturing and moving poorly fixed granular particles by a water stream, particularly powdery particles, including their agglomeration, which will result in low efficiency of liquid purification during the life of the claimed filtering element.